Making Tholins

Cassini Spacecraft Reveals Evidence of Tholin Formation at High Altitudes in Titan’s Atmosphere

Scientists have long known that the lower atmosphere of Saturn’s moon Titan contains organic aerosols, or tholins, formed from simple organic molecules, such as methane and nitrogen. Researchers had assumed these tholins formed at altitudes of several hundred kilometers, but new information gathered by three particle spectrometers aboard the Cassini spacecraft shows tholin formation happens in Titan’s atmosphere at altitudes greater than 1,000 kilometers. The results also show tholins form differently than previously thought.

Scientists at Southwest Research Institute (SwRI), the University of Kansas, University College London and The University of Texas at San Antonio report results of the observations in the paper “The Process of Tholin Formation in Titan’s Upper Atmosphere,” published in the May 11 issue of Science.

“Tholins are very large, complex organic molecules thought to include chemical precursors to life,” said Dr. Hunter Waite, an Institute scientist in SwRI’s Space Science and Engineering Division, and leader of Cassini’s Ion Neutral Mass Spectrometer (INMS) team. “Understanding how they form could provide valuable insight into the origin of life in the solar system.”

During Cassini’s first encounters with Titan, the INMS revealed an atmosphere dominated by nitrogen and methane. Of significance from measurements taken in later flybys, however, was the detection of benzene, a critical component in the formation of aromatic hydrocarbon compounds. At the same time, two other Cassini sensors that are part of the Cassini Plasma Spectrometer (CAPS) investigation, the Ion Beam Spectrometer (IBS) and Electron Spectrometer (ELS), measured large positive and negative ions.

The negative ions were a complete surprise,” said Dr. David Young, also an SwRI Institute scientist, and leader of the CAPS investigation. “This suggests they may play an unexpected role in making tholins from carbon-nitrogen precursors.”

“An additional surprising point is the large numbers of negative ions we see during Cassini’s lowest flybys above the surface,” said Dr. Andrew Coates, a researcher at the Mullard Space Science Laboratory, University College London. “This newly discovered, and important, population represents a highly significant proportion of the whole ionosphere at these locations.”

“Our analysis suggests that the organic compounds are formed through ion-neutral chemical processes, which then give rise to the complex negative ions found by the ELS,” Waite added.